The invention relates to new and useful improvements in wireless data transmission systems. More particularly, the invention relates to systems for transmitting data between a stationary unit and mobile data carriers.
In the field of identification technology, data are often transmitted inductively, without contact, between a read-write unit, which is usually fixed, and mobile data memories. The read-write unit builds up a radio-frequency electromagnetic field for this purpose. It uses this on the one hand to transmit data to or receive data from the mobile data carrier. On the other hand, the field also transmits a great deal of electrical energy inductively to a mobile data carrier, in order to put it into a state in which it is ready to transmit or receive. A separate power supply for the data carrier, for example a battery, then becomes unnecessary.
This sort of transmission of data and energy requires at least that the mobile data memory be brought into the transmitting and receiving region of the read-write unit, i.e., into the unit's radio-frequency induction field.
Problems can arise if a person has to handle the mobile data memory in the form of an identity card, or if it is fastened to his/her clothing. Such applications occur, for example, in the case of mobile data memories which are used to permit automatic contactless access of people to public transport, leisure facilities, employment premises, etc. Specifically, the persons handling such cards may include individuals with medical implants, such as a pacemaker, or with insulin pumps or the like. It cannot be entirely ruled out that radio-frequency alternating electromagnetic fields emitted by contactless identification devices of the above type may cause undesired impairment of the operation of the implanted appliance, should an implantee approach too close to a read-write unit.